Carbohydrate Metabolism Practice Questions

Q: True regarding glucose tolerance test are all except:

Can be done in fasting as well as post prandial state. The Oral Glucose Tolerance Test (OGTT) is a standardized diagnostic tool used to assess the body's ability to metabolize glucose. **Explanation of the Correct Answer:** **Option A** is the "except" statement because an OGTT **cannot** be performed in a post-prandial state. For the test to be valid and reproducible, the patient must be in a **fasting state** (at least 8–12 hours of overnight fasting). Administering a glucose load to a patient who has already eaten would lead to inaccurate results, as baseline insulin and glucose levels would already be fluctuating. **Analysis of Other Options:** * **Option B:** In pediatric cases, the standard dose is **1.75 g/kg** of body weight (up to a maximum of 75g). In adults, while a flat 75g dose is standard, the physiological basis remains rooted in weight-based titration for specific populations. * **Option C:** The **2-hour post-load glucose level** is the diagnostic gold standard for OGTT. It measures the body's efficiency in returning blood sugar to baseline via insulin action. * **Option D:** OGTT is a definitive diagnostic test for Diabetes Mellitus and Impaired Glucose Tolerance (IGT), especially when fasting plasma glucose is inconclusive. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Standard Dose:** 75 grams of anhydrous glucose dissolved in 250–300 ml of water, consumed within 5 minutes. * **Diagnostic Thresholds (2-hr value):** * Normal: <140 mg/dL * Impaired Glucose Tolerance (IGT): 140–199 mg/dL * Diabetes Mellitus: ≥200 mg/dL * **Dietary Preparation:** The patient should be on an unrestricted carbohydrate diet (at least 150g/day) for 3 days prior to the test to avoid "starvation ketosis" which can cause false-positive results. * **Cortisane-Stressed GTT:** Used to detect latent diabetes.

Q: Which glycogen storage disease causes hypoglycemia, hepatomegaly, growth retardation, muscle weakness, and accumulation of limit dextrins?

Cori's disease. **Explanation:** The clinical presentation of hypoglycemia, hepatomegaly, and growth retardation is characteristic of several Glycogen Storage Diseases (GSDs). However, the presence of **limit dextrins** (abnormally short outer branches of glycogen) and **muscle weakness** specifically points to **Cori’s disease (GSD Type III)**. 1. **Why Cori’s Disease is Correct:** It is caused by a deficiency of the **Debranching enzyme** (α-1,6-glucosidase). Without this enzyme, glycogen can be broken down by phosphorylase only until it reaches a branch point, leaving behind "limit dextrins." Unlike Von Gierke’s, Cori’s involves both the liver and muscles, explaining the myopathy/muscle weakness. Gluconeogenesis remains intact, so hypoglycemia is generally milder than in Type I. 2. **Why Other Options are Incorrect:** * **Von Gierke’s disease (Type I):** Caused by Glucose-6-Phosphatase deficiency. It presents with severe hypoglycemia, lactic acidosis, and hyperuricemia, but **no muscle involvement** and no limit dextrins. * **Andersen disease (Type IV):** Caused by **Branching enzyme** deficiency. It results in long, unbranched glucose chains (amylopectin-like). It typically presents with infantile cirrhosis and liver failure, not hypoglycemia. * **Pompe’s disease (Type II):** Caused by Lysosomal acid maltase deficiency. It primarily affects the heart (cardiomegaly) and muscles. Blood glucose levels are typically **normal**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "ABCD" – **A**ndersen is **B**ranching; **C**ori is **D**ebranching. * **Limit Dextrins:** Pathognomonic for Cori’s disease. * **Muscle Involvement:** If a GSD looks like Von Gierke’s but includes muscle weakness/wasting, think Cori’s (Type III). * **Type I vs. Type III:** Type I has elevated lactate; Type III has normal lactate levels.

Q: Which enzyme is specific for gluconeogenesis?

Glucose-6-phosphatase. **Explanation:** Gluconeogenesis is the metabolic pathway that results in the generation of glucose from non-carbohydrate precursors. While many steps of gluconeogenesis are simply the reverse of glycolysis, there are **three irreversible steps** in glycolysis that must be bypassed by four specific gluconeogenic enzymes. **1. Why Glucose-6-phosphatase is correct:** Glucose-6-phosphatase is one of the four key "bypass enzymes" of gluconeogenesis. It catalyzes the conversion of Glucose-6-phosphate to free Glucose, bypassing the irreversible hexokinase/glucokinase reaction of glycolysis. This enzyme is primarily located in the **lumen of the endoplasmic reticulum** of the liver and kidney, allowing these organs to release free glucose into the blood. **2. Why the other options are incorrect:** * **Aldolase (B):** This enzyme is involved in both glycolysis (cleaving Fructose-1,6-bisphosphate) and gluconeogenesis (condensing DHAP and Glyceraldehyde-3-phosphate). Because it functions in both directions, it is not "specific" to gluconeogenesis. * **Phosphoglycerate kinase (C) & Phosphoglycerate mutase (D):** These are reversible enzymes shared by both the glycolytic and gluconeogenic pathways. **High-Yield Clinical Pearls for NEET-PG:** * **The Four Unique Gluconeogenic Enzymes:** 1. Pyruvate carboxylase, 2. PEP carboxykinase (PEPCK), 3. Fructose-1,6-bisphosphatase (Rate-limiting step), and 4. Glucose-6-phosphatase. * **Von Gierke Disease (GSD Type I):** Caused by a deficiency of Glucose-6-phosphatase. It presents with severe fasting hypoglycemia, hepatomegaly, and hyperuricemia. * **Location:** Muscle lacks Glucose-6-phosphatase; therefore, muscle glycogen cannot contribute directly to blood glucose levels.

Q: All of the following tests can help in the diagnosis of Galactosemia EXCEPT?

Guthrie's test. **Explanation:** The correct answer is **Guthrie’s test** because it is a specific screening tool for **Phenylketonuria (PKU)**, not Galactosemia. It is a bacterial inhibition assay that detects elevated levels of phenylalanine in the blood using *Bacillus subtilis*. **Analysis of Options:** * **GALT Assay (Option A):** This is the **gold standard** for diagnosing Classical Galactosemia. It measures the activity of the enzyme *Galactose-1-phosphate uridyltransferase* in erythrocytes. Low or absent activity confirms the diagnosis. * **Mutation Analysis (Option B):** Molecular genetic testing (e.g., identifying mutations in the *GALT* gene like Q188R) is used for definitive diagnosis, carrier screening, and prenatal testing. * **Benedict’s Test (Option C):** This is a non-specific screening test for **reducing substances** in the urine. Since galactose is a reducing sugar, a child with galactosemia will show a positive Benedict’s test but a **negative Glucose Oxidase (Dipstick) test**, providing a strong clinical clue. **High-Yield Clinical Pearls for NEET-PG:** * **Classical Galactosemia** is an autosomal recessive deficiency of **GALT**. * **Clinical Triad:** Oil-drop cataracts, hepatosplenomegaly (jaundice), and intellectual disability. * **Early Sign:** Increased susceptibility to **E. coli sepsis** in the neonatal period. * **Management:** Immediate withdrawal of lactose/galactose from the diet (switch to soy-based formula). * **Duarte Variant:** A milder form of the disease with partial enzyme activity.

Q: A 17-year-old with type 1 diabetes presents with diabetic ketoacidosis after a weekend camping trip. He is noted to be jaundiced, and the plasma has a red color. Which of the following is a product of the enzyme for which this patient has a deficiency?

NADPH. ### Explanation **1. Why NADPH is the Correct Answer:** The clinical presentation describes a patient with **G6PD (Glucose-6-Phosphate Dehydrogenase) deficiency** triggered by **Diabetic Ketoacidosis (DKA)**. In G6PD deficiency, oxidative stress (caused here by acidosis and infection/stress from the trip) leads to the oxidation of hemoglobin into **Heinz bodies**, resulting in acute hemolysis (jaundice and red plasma/hemoglobinemia). G6PD is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. It catalyzes the conversion of Glucose-6-Phosphate to 6-Phosphogluconolactone, and in the process, it reduces NADP+ to **NADPH**. NADPH is essential for maintaining the pool of **reduced glutathione**, which protects erythrocytes from oxidative damage by neutralizing free radicals and hydrogen peroxide. **2. Why Other Options are Incorrect:** * **A. FADH2:** Produced primarily in the TCA cycle (Succinate dehydrogenase) and Beta-oxidation. It is not a product of the G6PD enzyme. * **B. NAD+:** This is an oxidized coenzyme. The HMP shunt is focused on producing *reduced* coenzymes (NADPH). * **C. NADH:** Produced during glycolysis and the TCA cycle. While it is a reducing equivalent, it is not the product of the HMP shunt and cannot be used by Glutathione Reductase to protect against oxidative stress in RBCs. **3. Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked recessive (more common in males). * **Triggers:** Fava beans, Infections, DKA, and Drugs (AAA: **A**ntimalarials like Primaquine, **A**ntibiotics like Sulfa drugs, **A**spirin/NSAIDs). * **Morphology:** **Heinz Bodies** (denatured Hb) and **Bite Cells** (formed by splenic macrophages removing Heinz bodies). * **Diagnosis:** Enzyme assay (Note: Levels may be falsely normal during an acute hemolytic episode as young reticulocytes have higher enzyme levels).

Q: Enantiomers are isomers that differ in structure at which carbon?

Penultimate Carbon. ### Explanation **Concept Overview:** Enantiomers are a specific type of stereoisomer that are **non-superimposable mirror images** of each other. In carbohydrate chemistry, the classification of a sugar as a **D-isomer** or **L-isomer** is determined by the orientation of the hydroxyl (-OH) group on the **penultimate carbon** (the chiral carbon furthest from the carbonyl group). **Why Option C is Correct:** The penultimate carbon (the "next-to-last" carbon) is the reference point for stereoisomerism in sugars. For example, in a 6-carbon glucose molecule, this is **C-5**. If the -OH group on this carbon is on the right, it is a D-isomer; if on the left, it is an L-isomer. Since D and L forms are mirror images of each other, they are enantiomers. **Analysis of Incorrect Options:** * **A. Last Carbon:** The last carbon (e.g., C-6 in glucose) is usually a primary alcohol group (CH₂OH) and is **achiral** (not bonded to four different groups), so it cannot determine isomerism. * **B. First Carbon:** In aldoses, the first carbon is the aldehyde group. While it is functional, it is not the reference for D/L nomenclature. * **D. Carbonyl Carbon:** This is the functional group (C-1 in aldoses, C-2 in ketoses). When sugars cyclize, this becomes the **anomeric carbon**, giving rise to **anomers** (alpha and beta forms), not enantiomers. **NEET-PG High-Yield Pearls:** * **Most abundant form:** Most naturally occurring sugars in the human body are **D-isomers** (enzymes are stereospecific for D-sugars). * **Amino Acids:** Unlike sugars, naturally occurring amino acids are primarily **L-isomers**. * **Epimers:** Isomers differing at a single asymmetric carbon *other* than the penultimate carbon (e.g., Glucose and Galactose are C-4 epimers). * **Racemic Mixture:** An equimolar mixture of D and L enantiomers that shows no optical activity.

Q: The enzyme phosphofructo kinase-1 is strongly activated by which of the following?

Fructose 2,6 bisphosphate. **Explanation:** Phosphofructokinase-1 (PFK-1) is the **rate-limiting and key committed step** of Glycolysis, converting Fructose 6-phosphate to Fructose 1,6-bisphosphate. Its regulation is a high-yield topic for NEET-PG. **Why Fructose 2,6-bisphosphate (F2,6-BP) is correct:** F2,6-BP is the **most potent allosteric activator** of PFK-1. It is produced by the bifunctional enzyme PFK-2. F2,6-BP functions by increasing the affinity of PFK-1 for its substrate (Fructose 6-phosphate) and decreasing the inhibitory effect of ATP. This ensures that glycolysis proceeds rapidly even when energy levels are relatively high. **Analysis of Incorrect Options:** * **B. Adenosine Triphosphate (ATP):** ATP acts as an **allosteric inhibitor**. High ATP levels signal that the cell has sufficient energy, thus "turning off" glycolysis to conserve glucose. * **C. Citrate:** Citrate is an intermediate of the TCA cycle. High levels indicate that energy production is meeting demand, acting as an **allosteric inhibitor** of PFK-1. * **A. Cyclic AMP (cAMP):** In the liver, high cAMP (triggered by Glucagon) leads to the phosphorylation and inactivation of PFK-2, which **decreases** F2,6-BP levels, thereby inhibiting PFK-1 and glycolysis. **High-Yield Clinical Pearls for NEET-PG:** * **PFK-1 Inhibitors:** ATP, Citrate, and H+ ions (low pH/acidosis). * **PFK-1 Activators:** Fructose 2,6-BP and AMP. * **Insulin vs. Glucagon:** Insulin increases F2,6-BP (stimulating glycolysis), while Glucagon decreases it (stimulating gluconeogenesis). * **Tauri Disease (GSD Type VII):** Caused by a deficiency of the PFK-1 enzyme in muscles, leading to exercise intolerance and hemolysis.

Question 1

True regarding glucose tolerance test are all except:

Accepted Answer

Can be done in fasting as well as post prandial state

Answer

1 gram of glucose/kg body weight is administered

Answer

Glucose levels are checked after 2 hours

Answer

Diagnosis of diabetes mellitus can be established

Question 2

A young man finds that every time he eats dairy products he feels very uncomfortable. His stomach becomes distended. He develops gas and diarrhea frequently. These symptoms do not appear when he eats foods other than dairy products. Which of the following is the most likely enzyme in which this young man is deficient?

Accepted Answer

sucrase

Answer

alpha-amylase

Answer

beta-galactosidase

Answer

alpha-glucosidase

Question 3

Glycosylated hemoglobin in a normal pregnant lady should be less than:

Accepted Answer

6%

Answer

4%

Answer

5%

Answer

7%

Question 4

A 12-year-old intellectually disabled boy presents with short stature, protuberant abdomen with an umbilical hernia, and a prominent forehead. His vision is normal, and his parents are unaffected. What is the metabolic defect in this disorder?

Accepted Answer

Iduronate Sulfatase

Answer

L-Iduronidase

Answer

Aryl Sulfatase B

Answer

Beta-Glucuronidase

Question 5

Which glycogen storage disease causes hypoglycemia, hepatomegaly, growth retardation, muscle weakness, and accumulation of limit dextrins?

Accepted Answer

Cori's disease

Answer

Von Gierke's disease

Answer

Anderson disease

Answer

Pompe's disease

Question 6

Which enzyme is specific for gluconeogenesis?

Accepted Answer

Glucose-6-phosphatase

Answer

Aldolase

Answer

Phosphoglycerate kinase

Answer

Phosphoglycerate mutase

Question 7

All of the following tests can help in the diagnosis of Galactosemia EXCEPT?

Accepted Answer

Guthrie's test

Answer

GALT assay

Answer

Mutation analysis

Answer

Benedict's test

Question 8

A 17-year-old with type 1 diabetes presents with diabetic ketoacidosis after a weekend camping trip. He is noted to be jaundiced, and the plasma has a red color. Which of the following is a product of the enzyme for which this patient has a deficiency?

Accepted Answer

NADPH

Answer

FADH2

Answer

NAD+

Answer

NADH

Question 9

Enantiomers are isomers that differ in structure at which carbon?

Accepted Answer

Penultimate Carbon

Answer

Last Carbon

Answer

First Carbon

Answer

Carbonyl Carbon

Question 10

The enzyme phosphofructo kinase-1 is strongly activated by which of the following?

Accepted Answer

Fructose 2,6 bisphosphate

Answer

Cyclic AMP

Answer

Adenosine triphosphate

Answer

Citrate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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